On February 23, LeBron James tweeted with the hashtag #CriticalMassMiami: “On behalf of myself and Dwyane Wade, we hate that we missed out again because of this crazy NBA schedule! Miss u guys.”

Miami Heat fans may have been puzzled, but cyclists in the city knew exactly what the NBA’s biggest star was talking about. James and two teammates, guards Wade and Mario Chalmers, have become high-profile converts to Critical Mass, the group-ride movement that takes place each month in cities around the world. After hearing about the event from friends, Wade was the first to participate – last September, he and his girlfriend, actress Gabrielle Union, pedaled a 12-mile route. Two months later, James and Chalmers joined in for the 20-mile nighttime trek; James even posted a group photo to Instagram.

“Ever since my first ride, I’ve been hooked,” says Wade. “It’s motivating to be around bikers who are dedicated and push themselves beyond the limit.”

Even though James has begun riding to and from Heat games (“I went without a car all day,” he recently bragged to Fox Sports), locals were taken aback when the Heat players arrived. “They never issued a media alert or anything,” says regular rider Rydel Deed. “They simply showed up and tried to remain as anonymous as possible.”

Since it kicked off with a sparsely attended group ride in San Francisco in 1992, Critical Mass has become the Occupy of the sports world – a leaderless yet loosely organized movement promoting anti-gas transportation and respect for cyclists. The gatherings have resulted in confrontations with police and drivers in some cities. But in Miami, they amount to a carnival on wheels, complete with biker-pulled sound systems. “It’s a nonstop party, but the message is, ‘Hi, we’re here, respect us on the road,'” says four-year veteran Daus Studenberg.

The Heat players’ motivations may be more athletic than political: “It gave my body a different type of conditioning challenge,” says Wade. “I had one of my best games of the season after a Critical Mass bike ride.” But no one’s denying that the presence of star athletes may be a tipping point for the movement. “They appeal to a larger audience,” says Deed, “and those who look up to them might get on a bicycle.”